Enhanced co-registered optical systems

ABSTRACT

An imaging optical system including a plurality of imaging optical sub-systems, each having at least one optical element and receiving light from a source, and a plurality of spectrometer optical sub-systems, each spectrometer optical sub-system receiving light from at least one of the imaging optical sub-systems, each imaging optical sub-system and spectrometer optical sub-system combination having a spatial distortion characteristic, each spatial distortion characteristic having a predetermined relationship to the other spatial distortion characteristics.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication Ser. No. 62/408,440, filed Oct. 14, 2016, entitled ENHANCEDCO-REGISTERED OPTICAL SYSTEMS and incorporated herein by reference inits entirety for all purposes.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with U.S. Government support from the U.S. Armyunder subcontracts R401-SC-20316-0252 and R401-SC-20316-0273 (PrimeW15P7T-06-D-R401) and subcontract WRI-002 (PO 22713, PrimeW909MY-12-D-0008/0012). The U.S. Government has certain rights in theinvention.

BACKGROUND

The present teachings relate to hyperspectral imaging sensors andparticularly to hyperspectral imaging sensors having two or morespectrometers that operate over different spectral bands.

In some applications, spectral algorithms that process data from acombination of both spectral bands are used. Each spectrometer of thehyperspectral imaging sensor that operates in a particular bandtypically has the individual keystone distortions corrected within itsparticular band.

In those cases, the magnification and spatial distortion differencesbetween those spectrometers typically differ by amounts greater than apixel in some portions of the spatial field, making the spectral purityof data for a given object in the scene insufficient to provide reliableresults over the combined spectral bands.

There is a need for hyperspectral imaging sensors having spectrometersthat operate over different spectral bands and have increased fidelityfor the exploitation of spectral algorithms over their combined spectralbands.

SUMMARY OF THE INVENTION

The embodiments of the present invention provide a hyperspectral imagingsensor with increased fidelity for the exploitation of spectralalgorithms.

More specifically, the embodiments disclose and describe a hyperspectralimaging optical system made up of at least two individual spectrometers,each operating over a substantially different spectral band (e.g.visible and long-wave infrared). Although systems of this type, boththose with common aperture and those with independent apertures, havebeen developed, each of the spectrometers typically contribute data thatis processed using algorithms tailored to each of the individualspectral bands. For those skilled in the art, the spectral smile andkeystone distortions of each sensor play a significant role in thesensor's ability to provide spectrally pure data for a given object inthe scene. For this reason, these distortions are typically limited to afraction of a pixel.

To increase the fidelity of the sensor and provide better targetdiscrimination, spectral algorithms that process data from a combinationof both spectral bands can be utilized. While the individual keystonedistortions of the two systems may be well corrected within themselves,the magnification and spatial distortion differences between the twospectrometer systems typically differ by amounts greater than a pixel insome portions of the spatial field, making the spectral purity of datafor a given object in the scene insufficient to provide reliable resultsover their combined spectral bands.

For a better understanding of the present invention, together with otherand further objects thereof, reference is made to the accompanyingdrawings and detailed description detailed description and its scopewill be pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an embodiment of the present invention,taken along its optical axis in the plane parallel to the direction ofdispersion;

FIG. 2 is a schematic view of another embodiment of the presentinvention, taken along its optical axis in the plane parallel to thedirection of dispersion; and

FIG. 3 is a graph of the overlaid spatial distortion characteristics ofthe two sensors in the embodiment of the present invention described inFIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention relates to novel optical designs, which provide greaterspectral fidelity and performance than previous designs.

More specifically, the embodiments disclose and describe a hyperspectralimaging optical system made up of at least two individual spectrometers,each operating over a substantially different spectral band (e.g.visible and long-wave infrared). Although systems of this type, boththose with common aperture and those with independent apertures, havebeen developed, each of the spectrometers typically contribute data thatis processed using algorithms tailored to each of the individualspectral bands. For those skilled in the art, the spectral smile andkeystone distortions of each sensor play a significant role in thesensor's ability to provide spectrally pure data for a given object inthe scene. For this reason, these distortions are typically limited to afraction of a pixel.

To increase the fidelity of the sensor and provide better targetdiscrimination, spectral algorithms that process data from a combinationof both spectral bands can be utilized. While the individual keystonedistortions of the two systems may be well corrected within themselves,the magnification and spatial distortion differences between the twospectrometer systems typically differ by amounts greater than a pixel insome portions of the spatial field, making the spectral purity of datafor a given object in the scene insufficient to provide reliable resultsover their combined spectral bands.

Reference is made to FIG. 1, which is a schematic view of an embodimentof the present invention 100. (Exemplary embodiments of components usedin FIG. 1 can be found in U.S. Pat. No. 9,885,606, corresponding to U.S.patent application Ser. No. 14/212,327, entitled COMPACT SPECTROMETERWITH HIGH SPECTRAL RESOLUTION, and filed on Mar. 14, 2014, and U.S. Pat.No. 9,568,737, entitled COMPACT COMMON APERTURE IMAGER SYSTEM, issued onFeb. 14, 2017, both of which are incorporated by reference herein intheir entirety and for all purposes.) Light from a source (not shown)located at the object plane (not shown) is incident upon an afocaloptical system 110, which is capable of substantially receiving thelight from the source and substantially collimating the light. Thissubstantially collimated light is then incident upon a beam splitter120, the preferred embodiment of which is, but not limited to, a planarbeam splitter, but in general is any method of separating light, byreflection, refraction, diffraction, transmission, or any combinationthereof, hereinafter referred to generally as a beam splitter, which iscapable of substantially transmitting a first portion of the light andcapable of substantially reflecting a second portion of the light. Thefirst portion of the light is then incident upon a first focusingoptical system 230, which is capable of substantially receiving thefirst portion of the light and substantially transmitting that light toa first spectrometer optical system 240. The first spectrometer opticalsystem 240 can be, without limitation, any spectrometer optical systemwhich substantially disperses and re-images a portion of the lightreceived from the first focusing optical system 230 to a focus position(hereinafter also referred to as an image plane) where a CCD array,phosphorescent screen, photographic film, microbolometer array, or othermeans of detecting light energy, hereinafter referred to generally as adetecting element 260, is located. (The image plane and the detectingelement have the same identifying number.) The second portion of thelight is incident upon a second focusing optical system 330 that iscapable of substantially receiving the second portion of the light andsubstantially transmitting that light to a second spectrometer opticalsystem 340. The second spectrometer optical system 340 can be, withoutlimitation, any spectrometer optical system which substantiallydisperses and re-images a portion of the light received from the secondfocusing optical system 330 to a focus position (hereinafter alsoreferred to as another image plane) where a detecting element 360 islocated.

Reference is made to FIG. 2, which is a schematic view of an embodimentof the present invention 200. A first portion of light from a source(not shown) located at the object plane (not shown) is incident upon afirst focusing optical system 230, which is capable of substantiallyreceiving the first portion of the light and substantially transmittingthat light to an optional first spectrometer optical system 240. Thefirst spectrometer optical system 240 can be, without limitation, anyspectrometer optical system which substantially disperses and re-imagesa portion of the light received from the first focusing optical system230 to a focus position of a detecting element 260. A second portion oflight from a source (not shown) located at the object plane (not shown)is incident upon a second focusing optical system 330 that is capable ofsubstantially receiving the second portion of the light andsubstantially transmitting that light to an optional second spectrometeroptical system 340. The second spectrometer optical system 340 can be,without limitation, any spectrometer optical system which substantiallydisperses and re-images a portion of the light received from the secondfocusing optical system 330 to a focus position of a detecting element360.

Reference is made to FIG. 3, which shows the spatial distortioncharacteristics of the combined first focusing optical system 230 andfirst spectrometer optical system 240 as a series of solid linesrepresenting different wavelengths in its spectral band and the combinedfirst focusing optical system 330 and first spectrometer optical system340 as a series of dashed lines representing different wavelengths inits spectral band. The variation within each of the individual seriesrepresents the keystone distortion of those individual sensors, whilethe overlap between the red and blue series represents the degree ofspatial distortion matching between the two hyperspectral systems.

In the embodiments illustrated in FIG. 1 and FIG. 2, the spatial imagingcharacteristics of the first and second focusing optical systems andhyperspectral optical systems are specifically designed to substantiallymatch each other across the spatial field. In one embodiment, this isaccomplished by designing the first optical system and then placingindividual restrictions on the spatial field characteristics of thesecond optical system during the design process, thereby requiring thespatial distortion characteristics of the second optical system tosubstantially match or substantially minimize the difference betweenthat of the first optical system. This can be accomplished, for exampleand without limitation, by varying the radii of curvature, thickness,refractive index, etc. of one or more of the optical elements in eithersystem while constraining the desired location of the image centroids atvarious spatial positions such that the difference between thedistortion characteristics of the two systems is substantially smallrelative to the size of the detecting elements of one or both systems.

For sufficient spectral purity for the application of spectralalgorithms that might use data across both spectral bands, this matchingwould typically be limited to less than a pixel on the detector orequivalently, focal plane array. In another embodiment, the two systemsmay be designed simultaneously with the constraints of matched spatialdistortion characteristics, where the distortion characteristics of thetwo systems do not differ substantially relative to the size of thedetecting elements of one or both systems, or to a specific desiredspatial distortion profile. In addition this spatial distortion matchingcan be used to balance the individual keystone distortions of theindividual system to further reduce the combined spatial/spectraldistortion of the system.

For the purposes of describing and defining the present teachings, it isnoted that the term “substantially” is utilized herein to represent theinherent degree of uncertainty that may be attributed to anyquantitative comparison, value, measurement, or other representation.The term “substantially” is also utilized herein to represent the degreeby which a quantitative representation may vary from a stated referencewithout resulting in a change in the basic function of the subjectmatter at issue.

Although the invention has been described with respect to variousembodiments, it should be realized this invention is also capable of awide variety of further and other embodiments within the spirit andscope of the invention. For example, although the embodiment shownutilizes a common aperture design, it should be noted that the matchingof the spatial distortion characteristics between one or more sensors isnot restricted to systems with common apertures and/or entrance pupils,and can be applied to those systems having two or more independentapertures as well. It should additionally be noted that the spectrometersystems do not need to have the same focal lengths, fields of view,apertures, pixel sizes, or instantaneous fields of view as each other tostill benefit from the matched or minimized difference in spatialdistortion described in this invention. This invention is not limited tooptical systems having only two spectrometers but also applies tooptical systems have a plurality of spectrometers of spectral bands.Furthermore, any number of optical elements, reflective or refractive,comprising without limitation refractive, reflective, and/or diffractiveelements, can be used in the embodiments of the present invention, andany aspects of the embodiments of the present invention, including butnot limited to those shown, can be used in combination with one anotheras still further embodiments.

The invention claimed is:
 1. An imaging optical system comprising: afirst imaging optical sub-system having at least one optical element;said first imaging optical sub-system being optically disposed toreceive light from a source; a first spectrometer optical sub-systemhaving at least one optical element; said first spectrometer opticalsub-system configured to substantially receive light from said firstimaging optical sub-system; said first spectrometer optical sub-systembeing configured to disperse said light received from said first imagingoptical sub-system and imaging said dispersed light onto a first imageplane; said first imaging optical sub-system and said first spectrometeroptical sub-system having a combined first spatial distortioncharacteristic; a second imaging optical sub-system having at least oneoptical element; said second imaging optical sub-system being opticallydisposed to receive light from a source; and a second spectrometeroptical sub-system having at least one optical element; said secondspectrometer optical sub-system configured to substantially receivelight from said second imaging optical sub-system; said secondspectrometer optical sub-system being configured to disperse said lightreceived from said second imaging optical sub-system and imaging saiddispersed light onto a second image plane; said second imaging opticalsub-system and said second spectrometer optical sub-system having acombined second spatial distortion characteristic; said first spatialdistortion characteristic and said second spatial distortioncharacteristic being substantially matched.
 2. The optical imagingsystem of claim 1 wherein said second imaging optical sub-system andsaid second spectrometer optical sub-system are designed tosubstantially minimize the difference between said first and said secondspatial distortion characteristics.
 3. The optical imaging system ofclaim 1 wherein at least one optical element is refractive.
 4. Theoptical imaging system of claim 1 wherein at least one optical elementis reflective.
 5. The optical imaging system of claim 1 furthercomprising: at least one detecting element located substantially at saidfirst image plane.
 6. An imaging optical system comprising: an opticalsub-system having at least one optical element; a beam splitting device;said beam splitting device being optically disposed to receive lightfrom said optical sub-system; a first imaging optical sub-system havingat least one optical element; said beam splitting device beingconfigured to substantially direct a first portion of the light to saidfirst imaging optical sub-system; said first imaging optical sub-systembeing optically disposed to receive light from said beam splittingdevice; a first spectrometer optical sub-system having at least oneoptical element; said first spectrometer optical sub-system configuredto substantially receive light from said first imaging opticalsub-system; said first spectrometer optical sub-system being configuredto disperse said light received from said first imaging opticalsub-system and imaging said dispersed light onto a first image plane;said first imaging optical sub-system and said first spectrometeroptical sub-system having a combined first spatial distortioncharacteristic; a second imaging optical sub-system having at least oneoptical element; said beam splitting device being configured tosubstantially direct a second portion of the light to said secondimaging optical sub-system; said second imaging optical sub-system beingoptically disposed to receive light from said beam splitting device; anda second spectrometer optical sub-system having at least one opticalelement; said second spectrometer optical sub-system configured tosubstantially receive light from said second imaging optical sub-system;said second spectrometer optical sub-system being configured to dispersesaid light received from said second imaging optical sub-system andimaging said dispersed light onto a second image plane; said secondimaging optical sub-system and said second spectrometer opticalsub-system having a combined second spatial distortion characteristic;said first spatial distortion characteristic and said second spatialdistortion characteristic being substantially matched.
 7. The opticalimaging system of claim 6 wherein said second imaging optical sub-systemand said second spectrometer optical sub-system are designed tosubstantially minimize the difference between said first and said secondspatial distortion characteristics.
 8. The optical imaging system ofclaim 6 wherein at least one optical element is refractive.
 9. Theoptical imaging system of claim 6 wherein at least one optical elementis reflective.
 10. The optical imaging system of claim 6 furthercomprising: at least one detecting element located substantially at saidfirst image plane.
 11. An imaging optical system comprising: a pluralityof imaging optical sub-systems having at least one optical element; saidplurality of imaging optical sub-systems being optically disposed toreceive light from a source; a plurality of spectrometer opticalsub-systems having at least one optical element; each spectrometeroptical sub-system of said plurality of spectrometer optical sub-systemsconfigured to substantially receive light from at least one imagingoptical sub-system of said plurality of imaging optical sub-systems; aplurality of image planes; each spectrometer optical sub-system of saidplurality of spectrometer optical sub-systems being configured todisperse said light received from said plurality of imaging opticalsub-systems and imaging said dispersed light onto at least one imageplane of said plurality of image planes; a plurality of spatialdistortion characteristics; each imaging optical sub-system andspectrometer optical sub-system of said plurality of imaging opticalsub-systems and said plurality of spectrometer optical sub-systemshaving a spatial distortion characteristic of said plurality of spatialdistortion characteristics; each spatial distortion characteristic ofsaid plurality of spatial distortion characteristics being substantiallymatched.
 12. A method for providing spectral fidelity and improvingperformance in sensors that operate in at least two spectral bands, themethod comprising: configuring a first spectrometer, the firstspectrometer operating in a first spectral band, to have a first spatialdistortion characteristic; and configuring a second spectrometer, thesecond spectrometer operating in a second spectral band, to have asecond spatial distortion characteristic; wherein the sensor comprisesthe first spectrometer and the second spectrometer; the firstspectrometer and the second spectrometer being configured such that thefirst spatial distortion characteristic and the second spatialdistortion characteristic have a predetermined relationship.
 13. Themethod of claim 12 wherein the first spatial distortion characteristicand a second spatial distortion characteristic are substantiallymatched.
 14. The method of claim 12 wherein a difference between thefirst and second spatial distortion characteristics is substantiallyminimized.